1. Field of the Invention
This invention relates to the manufacture of flat glass by processes in which glass is supported on a pool of molten metal during forming. More particularly, this invention relates to dam barriers for placement in a glass-supporting pool of molten metal for influencing flows of molten metal within such a pool.
2. Brief Description of the Prior Art
A number of patents describe the use of devices for segregating one portion of a glass-supporting pool of molten metal from another portion of the glass-supporting pool of molten metal in order to influence the flows of molten metal therein and consequently to influence the temperature of molten metal upon which glass is supported during forming. Beginning with U.S. Pat. No. 789,911 to Hitchcock, which issued in 1905, it has been appreciated that barriers could be employed for separating a glass-supporting pool of molten metal into a plurality of sections. As shown in U.S. Pat. No. 3,432,284 to Gulotta and U.S. Pat. No. 3,485,617 to Lawrenson et al., fixed dam barriers of various designs have been employed to influence thermal conditions within a pool of glass-supporting molten metal during the formation of glass thereon.
U.S. Pat. No. 3,930,829 to Sensi illustrates dam barriers which are movable, submersible dam barriers that can be readily placed in an operating glass forming chamber without disruption of the manufacture of glass and may be manipulated to desired positions in order to establish particularly desired thermal conditions for the manufacture of glass of any desired thickness. The movable, submersible dam barriers disclosed by Sensi each comprise an inner body of dense material in order to insure their submersibility and an outer cover or shell of material which may serve as a protective cover for the inner body depending upon the material employed as an inner body. The present invention comprises an improvement to movable, submersible dam barriers such as those disclosed by Sensi.
It has been found that placement of movable, submersible dam barriers in a glass forming chamber at locations within the chamber where the molten metal temperature is above about 1500.degree. F. (820.degree. C.) yields some bubbling associated with such dam barriers. It is not fully known whether this bubbling is caused by the accumulation of gases dissolved in the molten metal on nucleation sites on a dam barrier and the periodic release of gas bubbles which have grown to sufficient size to separate from the surface upon which they nucleate or if the occurrence of bubbles associated with tin dam barriers employed at locations of extremely high temperature in a forming chamber is due to the discharge of gases from the materials from which the dam barriers are constructed. In either event, the present invention contemplates a method and apparatus improvement which will prevent or minimize the occurrence of bubbles associated with or attributed to the presence of a dam barrier in molten metal within a glass forming chamber at any location throughout such a chamber, including locations at which the molten metal temperature is above 1500.degree. F. (820.degree. C.).